It is known that natural gas can be transferred in the form of vapor or liquid via specific interfaces between a ship and a gas terminal.
FIG. 7 is a simplified diagram of a prior art gas unloading circuit.
Liquefied gas 101 transported in a tank 102 of a ship 100 is discharged to a tank 104 of a gas terminal 106 by a pump 108 immersed in the tank 102 of the ship 100. The liquefied gas is delivered into the tank 104 of the gas terminal via a circuit 110 including a loading arm 112 to allow for a certain amount of freedom of movement between the ship 100 and the gas terminal 106.
Depending on the feed requirement of a gas pipeline 114 fed with gas at high pressure, the liquefied gas 101 contained in the tank 104 of the gas terminal 106 is transferred by a pump 116 immersed in the tank 104 to a condenser 118. Thereafter, a high pressure pump 120 delivers the liquefied gas 101 to a heat exchanger 122 so as to cause it to be re-gasified prior to being delivered at substantially ambient temperature to the pipeline 114.
In addition, the gas ceiling 124 inside the tank 102 and the gas ceiling 126 inside the tank 104 are interconnected by a low pressure natural gas vapor circuit 128 via an arm 130. This enables the pressure in the two gas ceilings 124 and 126 to be controlled.
Heat entering into the tanks 102 and 104 causes the liquefied gas to evaporate, leading to excess natural gas vapor. A portion of the excess vapor can be taken from the circuit 128 by a low pressure compressor 132 which delivers it via a circuit 133 to the condenser 118 where it is reliquefied.
Furthermore, in order to ensure that the liquefied natural gas is re-gasified in the heat exchanger 122, sea water 123 is taken by a pump 134 via a circuit 135, and possibly heated using immersed burners 136 fed with natural gas vapor by a circuit 138, prior to being returned to the sea 123.
However, that transfer architecture consumes a large amount of gas in pure loss, leading to ecological harm as well as to a loss of revenue.
To solve that kind of problem, there exist gas terminals including installations for co-generation of energy, but they are very expensive.
Finally, while unloading its cargo, the ship 100 takes in sea water 123 via a circuit 140 to fill its ballast tanks 142 so as to ensure that the draft and the trim of the ship 100 are appropriate.
FIG. 8 shows a simplified diagram of a prior art liquefied gas loading circuit. In this example, the liquefied gas produced by a liquefaction plant (not shown) is conveyed via a circuit 144 into a storage tank 104 of a gas terminal 106 from which it is taken by an immersed pump 146 for sending via a circuit 110 to a tank 102 on board a ship 100.
Similarly, a compressor 132 extracts, from a circuit 128, excess natural gas vapor generated by ingress of heat, and delivers it to the liquefaction plant.
Finally, while loading its cargo, the ship 100 empties out the sea water that it initially contained in its ballast tank 142 via a circuit 140 so as to ensure that the draft and the trim of the ship 100 are appropriate.
The sea water used in the ballast tanks of the ship or in the heat exchangers used for re-gasifying liquid natural gas in the gas terminal leads to corrosion in the ballast tanks and in the heat exchangers.
In addition, sea water causes sediment to be deposited in the ballast tanks 142 which leads subsequently to large expense for its removal.